1. Field of the Invention
The invention relates to a hydrostatic drive for driving various types of vehicles.
2. Discussion of the Prior Art
A hydrostatic drive for driving diverse types of vehicles is known from DE-OS 20 26 910. In the case of the hydrostatic drive disclosed in the publication, a first hydraulic pump, a first hydraulic motor, a second hydraulic pump and a second hydraulic motor are arranged in series in a working circuit. When the two hydraulic motors drive different vehicle wheels, the problem arises that, in the event of a slip at one of the vehicle wheels, the hydraulic motor connected to this vehicle wheel does not contribute to the driving of the vehicle and the efficiency of the hydrostatic drive is therefore reduced.
A further hydrostatic drive is known from EP 0 547 947 A1. In the case of the hydrostatic drive disclosed in this publication, two vehicle wheels lying opposite on a vehicle axle are driven in each case by two hydraulic motors arranged in pairs on a common shaft. The hydraulic fluid delivered in a working circuit by a hydraulic pump branches upstream of the hydraulic motors arranged in pairs. Whereas the hydraulic fluid from the outlet of one of the two hydraulic motors arranged in pairs flows back directly to the hydraulic pump, the outlet of the other hydraulic motor arranged on the same shaft is connected to the hydraulic pump via a further hydraulic motor in each case, these further hydraulic motors driving vehicle wheels of another vehicle axle. In the case of the hydrostatic drive disclosed in this publication, no measures are provided to prevent the efficiency of the drive being considerably reduced in the event of a slip at one of the vehicle wheels.
EP 0 505 254 A1 discloses a hydrostatic drive in which all the hydraulic motors driving different vehicle wheels are connected in parallel to the hydraulic pump. Speed sensors are provided at the output shafts of the individual hydraulic motors. As a function of the speeds determined at the individual output shafts, the amount of pressure fluid flowing through the assigned hydraulic motors can be regulated by adjustable, throttled branch valves, so that possible speed differences are equalised and in particular steering or exact straight-line driving is permitted. However, this arrangement has only limited use for equalising a slip at one of the vehicle wheels.
EP 0 378 742 A2 discloses a hydrostatic drive in which a first and second drive train are completely separated from each other on cornering, the first drive train having a first hydraulic pump and a first hydraulic motor and the second drive train having a second hydraulic pump and a second hydraulic motor. In order to permit as exact a straight-line driving as possible, the hydraulic motors can be mechanically connected to each other on the one hand by means of a mechanical coupling on straight-line driving. On the other hand, the separated hydraulic working circuits are hydraulically connected to each other by valves on straight-line driving. A measure for preventing the efficiency being impaired in the event of a slip at one of the two drive trains is not disclosed in this publication.
The object on which the invention is based is to provide a hydrostatic drive for driving a plurality of drive trains, in which the efficiency is not substantially reduced if a slip occurs at one of the drive trains.
The invention is based on the finding that by means of a switching valve which in the event of a slip at one of the drive trains bypasses the hydraulic motor connected to this drive train, a reduction of the efficiency can be countered by then supplying the hydraulic fluid directly to the respective other hydraulic motor not affected by the slip. The full hydraulic power of the hydraulic pump is thus available to the hydraulic motor not affected by the slip.
Advantageous developments of the invention are disclosed in the subclaims.
It is advantageous to assign a switching valve to each hydraulic motor, so that when a slip occurs at one of the drive trains the hydraulic motor assigned to this drive train can be bypassed by the assigned switching valve.
The switching valves can be driven by the pressure drop at the hydraulic motor assigned to the respective switching valve or else by the pressure drop at the other hydraulic motor or hydraulic motors. One criterion for the occurrence of a slip at a drive train is either a marked reduction of the pressure drop at the hydraulic motor assigned to this drive train or a marked increase of the pressure drop at the other hydraulic motor or hydraulic motors.
Alternatively, the occurrence of a slip can also be detected by comparing the speeds at the different drive trains. If the different drive trains drive different wheels of the vehicle, which, given an identical wheel diameter, rotate at an identical speed in the event of a slip-free drive, a deviation of the speeds at the differently driven vehicle wheels indicates an occurrence of a slip at that vehicle wheel which has a markedly higher speed than the other vehicle wheels.